The present invention claims priority to Japanese Application No. P2000-251241, filed Aug. 22, 2000, which application is incorporated herein by reference to the extent permitted by law.
The present invention relates to a solid-state imaging device and a process for production thereof, the solid-state imaging device being characterized in that each pixel has a convex lens with an upwardly curved surface which is embedded in an inter-layer dielectric between a light-receiving portion and an on-chip lens.
The CCD solid-state imaging device should desirably have a reduced chip size and an increased number of pixels. Unfortunately, this object is not achieved simply by reducing the chip size while leaving the current pixel size as it is because such an attempt ends up with a reduction in the number of pixels and hence a reduction in resolution. Nor is the object achieved simply by increasing the number of pixels while leaving the current pixel size as it is because such an attempt ends up with an increase in chip size and hence an increase in production cost and a decrease in yields.
Therefore, for reduction in chip size and for increase in the number of pixels, it is essential to reduce the pixel size from the current one. If this object is achieved, it is possible to provide a small-sized CCD imaging device which keeps the current resolution unchanged or conversely to improve resolution while keeping the current chip size unchanged.
The problem with reduction in pixel size is that the amount of light incident on a unit pixel decreases and the light-receiving portion of a unit pixel becomes poor in sensitivity characteristics. Although the second difficulty can be overcome by improving the photoelectric conversion efficiency with a concomitant adverse effect of amplifying noise components, the result is a decrease in S/N ratio of image signals output from the CCD imaging device. In other words, it is not desirable to compensate for the loss of sensitivity characteristics resulting from reduction in pixel size only with improvement in photo-electric conversion efficiency, but it is desirable to improve as much as possible the condensing efficiency of each pixel, thereby preventing the decrease in S/N ratio.
An idea contrived from the above-mentioned standpoint is to form an on-chip lens (OCL) on the color filter formed on the light-receiving portion. This idea, however, is not practicable for a CCD imaging device having a pixel size smaller than 4xc3x974 xcexcm, because the efficiency of condensing light with an on-chip lens alone has almost approached the upper limit. A new technology to overpass the limit has been proposed in Japanese Patent Laid-open No. Hei 11-40787. This technology is concerned with an additional convex lens in filmy form of light-transmitting material which is formed in the layer between the on-chip lens and the light-receiving portion. This additional convex lens is designed to improve further the efficiency of condensing light. According to the disclosure, the convex lens is formed by the process which is explained below with reference to FIGS. 6A to 7C. This process is referred to as xe2x80x9cconventional process 1xe2x80x9d.
As shown in FIG. 6A, the process starts with fabrication of a silicon substrate 1 to form thereon the following components in the conventional way. A light-receiving portion 2, charge transfer portions 3xe2x88x921 and 3xe2x88x922, a gate portion (not shown) between the light-receiving portion 2 and the charge transfer portion 3xe2x88x921, and a channel stopper (not shown) between the light-receiving portion 2 and the charge transfer portion 3xe2x88x922. Transfer electrodes 5 are embedded in the insulating film 4 covering the charge transfer portions 3xe2x88x921 and 3xe2x88x922. On the insulating film 4 is formed a shielding film 6 of high-melting metal which has an opening above the light-receiving portion 2.
Then, a BPSG film 20 is formed on the shielding film 6 and the opening therein. The BPSG film 20 is planarized by reflowing as shown in FIG. 6B. On the planarized film 20 is formed a light-transmitting film 21a from silicon nitride (P-SiN) or silicon oxide (P-SiO2) by plasma CVD as shown in FIG. 6C.
The light-transmitting film 21a is coated with a resist. The resist film is patterned such that a region around the center of the light-receiving portion 2 remains. The patterned resist undergoes reflowing, so that it softens and forms a convex lens (resist pattern RP) having a prescribed curvature, as shown in FIG. 7A.
Etching is performed under the condition that the resist and the light-transmitting film have almost the same selectivity. Etching removes the resist, while leaving the light-transmitting film in the form of convex lens 21. The shape of the convex lens 21 conforms well to the shape of the resist pattern RP, as shown in FIG. 7B.
Subsequently, the convex lens 21 is embedded in a planarizing film 9. Finally, an on-chip color filter (OCCF) 10 and an on-chip lens (OCL) 11 are formed in the usual way, as shown in FIG. 7C.
There is another process (xe2x80x9cconventional process 2xe2x80x9d) in which the convex lens forming step in xe2x80x9cconventional process 1xe2x80x9d is modified as explained below.
FIGS 8A to 9C are sectional views showing xe2x80x9cconventional process 2xe2x80x9d.
As shown in FIG. 8A, xe2x80x9cconventional process 2xe2x80x9d starts with fabrication of a silicon substrate 1 to form the following components thereon as in xe2x80x9cconventional process 1xe2x80x9d. A light-receiving portion 2, charge transfer portions 3xe2x88x921 and 3xe2x88x922, an insulating film 4, transfer electrodes 5, and a shielding film 6.
A PSG film or BPSG film 22 is formed on the shielding film 6 and the opening therein. The PSG film or BPSG film undergoes reflowing. According to xe2x80x9cconventional process 2xe2x80x9d, this reflowing does not achieve complete planarizing but forms a depression 22a above the light-receiving portion 2.
As shown in FIG. 8B, a light-transmitting film 23a of P-SiN or P-SiO2 is formed on the PSG film or BPSG film 22. On the light-transmitting film 23a is formed resist R which is subsequently planarized.
Etching is performed under the condition that the resist R and the light-transmitting film 23a have almost the same selectivity. Etching forms the light transmitting film 23b, with its surface planarized, as shown in FIG. 8C.
As shown in FIG. 9A, a light transmitting film 23c is formed on the planarized surface. Then, a resist is applied to the light transmitting film 23c, and the resist film is patterned such that a region around the center of the light-receiving portion 2 remains. This patterning is following by reflowing. In this way there is obtained a resist pattern RP in the form of convex lens having a prescribed curvature.
Etching is performed again under the condition that the resist and the light-transmitting film have almost the same selectivity. Etching removes the resist, while leaving the light-transmitting film in the form of convex lens 23. The shape of the convex lens 23 conforms well to the shape of the resist pattern RP, as shown in FIG. 9B.
Finally, a planarizing film 9 is formed and an OCCF 10 and an OCL 11 are formed in the usual way, as shown in FIG. 9C.
The CCD imaging element obtained by xe2x80x9cconventional process 1xe2x80x9d and xe2x80x9cconventional process 2xe2x80x9d functions in the following way as shown in FIG. 10. The OCL 11 converges to some extent the incident ray (the vertical incident ray L0 and the oblique incident ray L1 with respect to the light-receiving plane). Another convex lens 21 (or 23) under the OCL 11 further converges the converged incident light. The converged light reaches the light-receiving portion 2. Thus, the convex lens 21 (or 23) improves the efficiency of condensing incident rays. The convex lens 21 (or 23) is particularly effective in condensing the oblique incident ray L1 indicated by broken lines in FIG. 10. This improves the sensitivity of each pixel.
xe2x80x9cConventional process 1xe2x80x9d and xe2x80x9cconventional process 2xe2x80x9d which are designed to form the convex lens 21 or 22 under the OCL 11 have the disadvantage that the curvature of the surface of the convex lens 21 or 23 depends on the curvature of the surface of the resist to be used as a mask during processing. In other words, the curvature of the resist surface should be large if the curvature of the lens surface is to be large, and the curvature of the resist surface should be small if the curvature of the lens surface is to be small. This makes it necessary to optimize the resist thickness so that the lens surface has a desired curvature.
Unfortunately, forming the convex lens by xe2x80x9cconventional process 1xe2x80x9d or xe2x80x9cconventional process 2xe2x80x9d poses a problem that the resist thickness tends to vary as it is reduced in proportion to the pixel size. Uneven resist coating implies that the thickness of the convex lens and the curvature of the lens surface vary in the same chip or in the same wafer. This in turn results in uneven sensitivity, that is, the level of output signal varies from one pixel to another.
A conventional practice to address this problem was to apply the resist somewhat thick enough to avoid coating variation in the step of forming the convex lens.
Thick coating means that the shape of the convex lens deviates from the optimal value in the case where the pixel size is small. In other words, the convex lens 21 has an excessively large curvature of lens surface relative to the pixel size, as shown in FIG. 11. The large curvature of the lens surface moves the focus upward from the light-receiving plane, as shown in FIG. 11, with the result that light diverges on the light-receiving plane and the amount of light received decreases. For the vertical light L0 indicated by solid lines in FIG. 11, the amount of light received decreases rather slightly.
However, the amount of light received decreases remarkably when the ratio of oblique light L1 (relative to OCL 11) increases. This occurs when the camera equipped with the CCD imaging element in question has its lens stopper opened more (so that the F value is small). As indicated by broken lines in FIG. 11, a large portion of the light passing through the convex lens strikes the shielding film 6 outside the light-receiving plane. This results in a considerable decrease in pixel sensitivity.
Moreover, the light incident aslant on the opening 6a of the shielding film 6 undergoes irregular reflection by interfaces of different films under the shielding film 6, and the reflected light enters the vertical transport portion 3xe2x88x921, thereby generating charges. The thus generated charges cause noise to the signal charges transferred from the vertical transfer portion 3xe2x88x921, and what is worse, this noise accumulates at each time of transfer and manifests itself as smear in the image.
In summary, the convex lens formed by the conventional process poses a problem that sensitivity decreases and smear readily occurs when the pixel is reduced.
On the other hand, any attempt to reduce the curvature of the convex lens despite small pixel size results in variation in resist thickness. This in turn causes the convex lenses to vary in shape, with the result that pixel sensitivity varies in the same element or from one element to another and hence sensitivity varies in the same image.
One way to overcome this problem in the conventional CCD imaging element is to slightly increase the resist thickness for the convex lens although it somewhat causes smear and decreases sensitivity due to oblique incident rays.
Consequently, there has been a demand for a new technology which solves the above-mentioned problem. Such a new technology should be able to eliminate smear and keeps sensitivity for oblique incident rays even though the convex lens under the OCL has a small curvature for uniform sensitivity.
It is an object of the present invention to provide a solid-state imaging device and a process for production thereof, the solid-state imaging device is characterized in that the convex lens has a small curvature for uniform sensitivity without loss in smear characteristics and decrease in sensitivity due to oblique incident rays.
The present invention is directed to an improved process for producing a solid-state imaging device including the steps of forming a light-receiving portion of a pixel in a region on the substrate surface, forming a convex lens with an upwardly curved surface which is embedded in an inter-layer dielectric above the light-receiving portion, and forming an on-chip lens above the convex lens, wherein the improvement includes forming sequentially the light-receiving portion, forming an inter-layer dielectric having a depression in its surface above the light-receiving portion, forming on the interlayer dielectric a light transmitting film having in its surface a concave conforming to the depression, forming at the position that covers the concave on the light transmitting film a mask layer with a convexly curved surface, and etching the mask layer and the light transmitting film all together, thereby making the light transmitting film into a shape of the convex lens.
Preferably, the process defined above further comprises, following the step of forming the light-receiving portion, the steps of forming electrodes to transfer charges generated by the light-receiving portion, the electrodes being positioned above both sides of the light-receiving portion and being insulated from the substrate, forming a shielding film which covers the step of the charge transfer electrodes and opens above the light-receiving portion, the shielding film being insulated from the charge transfer electrodes, and forming the inter-layer dielectric covering the shielding film and its opening in such a way that the depression is formed in the surface of the inter-layer dielectric in conformity with the step of the charge transfer electrode and the step of the shielding film.
Preferably, the process defined above further comprises, following the step of forming the light-receiving portion, the step of softening the inter-layer dielectric by heat treatment, thereby adjusting the depth of the depression.
Preferably, the process defined above further comprises the steps of forming a resist pattern (as the mask layer) on the light transmitting film and softening the resist pattern by heat treatment, thereby adjusting the curvature of the surface of the resist pattern.
Preferably, the process defined above includes etching which is carried out under the condition that the mask layer and the light transmitting film have almost the same selectivity.
Production of the solid-state imaging device by the process of the present invention offers the following advantage. The inter-layer dielectric has a depression and the light transmitting film has a concave which conforms to the depression. Owing to this structure, the resist applied to form a resist pattern on the light transmitting film can be made thicker than that in xe2x80x9cconventional process 1xe2x80x9d or xe2x80x9cconventional process 2xe2x80x9d in which a resist is applied onto a flat underlayer.
The consequence is that the curvature of the surface of the resist pattern can be made smaller than that in the conventional technology because an excess resist fills the concave when the resist is softened by heat treatment even though the resist thickness is large. Therefore, the solid-state imaging device produced by the process of the present invention differs from that produced by the conventional process in that the convex lens has a smaller curvature.
The process of the present invention needs only one step to form the convex lens from a single light transmitting film, whereas xe2x80x9cconventional process 2xe2x80x9d needs two steps to form the convex lens.
The present invention is directed to a solid-state imaging device having a light-receiving portion of a pixel formed in a region on the substrate surface, a convex lens with an upwardly curved surface which is positioned above the light-receiving portion and embedded in an inter-layer dielectric, and an on-chip lens formed above the convex lens, characterized in that the convex lens is formed on the depression in the surface of the underlying inter-layer dielectric such that the lower part of the lens which is made of the light transmitting material filling the depression is integral with the upper part of the lens which is made of the same light transmitting material as that of the lower part of the lens and has the convexly curved lens surface.